A well known practice for storing, and later delivering, an active ingredient involves forming the matter into compact tablets from which the matter can later be dissociated or released. One such use involves drug delivery where the drug is administered orally (or otherwise) into an organic system. Particularly, diffusion-controlled matrix devices have received a great deal of attention for drug delivery systems in past years. One approach has been to use insoluble porous disc matrices, in which the drug is loaded into the matrix to an amount greater than its solubility limit in the dissolution medium. It has been found, however, that the rate of solute diffusion out of these tablets is not consistent over time. Often, the diffusion rate changes with the square root of time. In such cases, the amount of drug available at a biologic site of absorption decreases quickly as a function of time if the absorption rate is greater than the drug release from the matrix, as it often is with the systems of the past.
This problem is significant in view of the fact that it is often desirable to have a steady rate of drug delivery. More specifically, a parabolic or zero order release kinetic is often desirable where drug delivery is desired to be constant over a given time period.
With much of the prior art, as the dissolution occurs, the surface area (i.e. the interface), between the solvent and active agent changes with time. If this dissolution begins at the outside surface of a tablet, as dissolution progresses the surface area between the active agent and the solvent decreases. Where a coated tablet has been drilled with a hole to allow solvent and active agent to interface beginning in the area where the hole was drilled, the surface area between the solvent and the active agent increases with time and thus the rate of dissolution increases with time. These problems continue to be the focus of attempts to achieve constant, or near constant, release of active agents into an organic body over time.
Simple monolithic tablets for extended release dosages have been fabricated by compressing a mixture of a water-insoluble polymer, a drug, and excipients. Such tablets may yield first-order kinetics or square-root-of-time kinetics. An inherent limitation of this type of monolithic matrix is the increase in diffusional length resistance over time due to the insolubility of the polymer.